1. Field of the Invention
The present invention relates to a method of manufacturing a composite ferrite used as a raw material for, for example, a magnetic member.
2. Description of the Related Art
Conventionally, the most popular method of synthesizing a composite oxide is the dry method in which a number of oxide powder materials are mixed with each other, and sintered at a high temperature to produce a composite oxide. The manufacturing procedure of this method includes the following steps.
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However, the dry method entails some problems such as (1) that it is difficult to mix the powder materials uniformly with each other, (2) that impurities are likely to be mixed into the materials while grinding, and (3) that it takes a long time to synthesize a composite oxide due to solid-phase diffusion between oxide particles.
As mentioned, it is difficult to obtain a uniform composite material and therefore the characteristics of the product made from such a material are unsatisfactory. Further, the productivity is extremely poor. It should be noted that if calcination, grinding, and mixing are repeated to make the material uniform, the productivity is further degraded due to such repetition.
In the meantime, there is a coprecipitation method used for producing a uniform composite oxide in which a composite oxide material is dissolved into a solution with a nitrate of the metal constituting the oxide material, and some alkali is added thereto so as to precipitate the uniform composite oxide by coprecipitation. The procedure of this method takes the following steps.
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A composite oxide produced by this method, in fact, exhibits a uniform composition; however the method requires some time-consuming steps such as filtration, washing and drying, and is a complicated process as a whole. Thus, the cost of the composite oxide produced by this method is extremely high.
Besides the above dry and coprecipitation methods, there is a spray roasting method for synthesizing a composite oxide, mainly, composite ferrite. The procedure of this method takes the following steps.
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Decomposition.fwdarw.Grinding.fwdarw.Product
In this method, the metal components of a ferrite is dissolved to make a chloride mixture solution, and the solution is sprayed and roasted in a combustion flame so as to synthesize a composite ferrite (Published Examined Japanese Patent Application (PEJPA) No. 47-11550).
The advantage of this method is that HCl, a byproduct, can be recycled for producing a metal chloride, thereby reducing the production cost. However, in the case where the method is applied to a solution containing metal chlorides having different vapor pressures, the metal chloride having the higher vapor pressure is volatilized out when atomizing, and dispersed in the form of chloride gas; therefore it is difficult to collect the oxide of the metal of the chloride having the higher vapor pressure (Published Unexamined Japanese Patent Application (PUJPA) No. 55-144421).
As a result, in order to manufacture a composite ferrite from metal chlorides containing chloride having high vapor pressure, it is necessary to perform mixing and roasting by the above dry method after the metal chloride having the high vapor pressure is transformed into oxides or carbonates.
In contrast thereto, there is proposed a method of atomizing a number of chloride solutions in the same direction as that of the flow of a high-temperature gas (Published Unexamined Japanese Patent Application Nos. 3-40921, and 4-192309). However, for large particles of solution mist, even when it is carried out in a roasting furnace by a combustion gas generated from an ordinary burner, roasting of the large particles do not proceed quickly, leaving some remaining chloride solution adhered to the furnace wall. When moderate roasting proceeds on the furnace wall, metal chlorides having high vapor pressure, for example, ZnCl.sub.2, of the remaining chlorides adhered, are selectively roasted. Then, ZnO created by the selective roasting is collected by the gas flow, increasing the concentration of ZnO in the collected material. As a result, a composite ferrite is produced which has a composition ratio significantly different from the composition concentration ratio of the solution.